In recent years, demand for introducing a next-generation optical transmission system which has transmission capacity of 40 Gbit/s or more has been increased as transmission traffic is increased. As a realizing method to introduce the next-generation optical transmission system, various modulation methods which have more excellent spectral efficiency, optical signal to noise ratio (OSNR) tolerance, and nonlinearity tolerance than those of a non return to zero (NRZ) modulation method which has been employed in a related art system have been employed. For example, as a modulation method of transmission capacity of 40 Gbit/s, a differential phase shift keying (DPSK) modulation method, a differential quadrature phase shift keying (DQPSK) modulation method, and the like are employed. Among the methods, two-bit data can be transmitted by one modulation of multi-level modulation in the DQPSK modulation method, so that the DQPSK modulation method has features such as high dispersion tolerance, high polarization mode dispersion (PMD) tolerance, and narrow spectrum and is expected as a modulation method of a next-generation optical transmission system. For example, modulation rate (baud rate) when a system obtains transmission capacity of 40 Gbit/s is 40 Gbit/s in the DPSK modulation method and 20 Gbit/s in the DQPSK modulation method.
In order to realize further capacity increase and property (OSNR tolerance, wavelength dispersion tolerance) improvement of the DQPSK modulation method, a modulation method in which a polarization multiplexing technique and a digital coherent receiving technique are combined has been actively developed in recent years. The polarization multiplexing technique is a technique to double the number of bits of modulation by modulating optical signals of respective polarization waves by different data signals. Combining the polarization multiplexing technique and the DQPSK modulation method enables four-bit transmission by one modulation, so that the combination has been regarded to be more likely applied to a 40 Gbit/s transmission system (baud rate 10 Gbit/s), a 100 Gbit/s transmission system (baud rate 25 Gbit/s), and the like.
FIGS. 1 to 2B illustrate a related art. As illustrated, signals of various modulation methods are mixed in an optical network system and modulation methods of signals of respective wavelengths are different from each other. Especially, if optical signals of modulation methods of which baud rates are different from each other are placed on adjacent wavelength positions, transmission performance is degraded. As a method for avoiding degradation of transmission performance, such method that a certain interval (guard band, refer to FIG. 1) is provided next to a wavelength position of an optical signal is employed. However, if optical signals of different modulation methods are arranged on random wavelength positions, a guard band has to be provided for each wavelength. Accordingly, a rate of guard bands in which no signals actually exist becomes large even in a signal band, degrading spectral efficiency. Therefore, in order to efficiently use a wavelength band, it is important to arrange optical signals of the same modulation method in a wavelength direction in a concentrated manner to reduce the number of guard bands in a signal band.
On the other hand, in an optical network system employing a reconfigurable optical add-drop multiplexer (ROADM) device, transmission paths are switched in response to request from a network controller 10, as illustrated in FIG. 2A. If the path switch is frequently performed, optical signals of various modulation methods are arranged in a random manner (fragmented) in the wavelength direction. Accordingly, the number of places on which optical signals of different modulation methods are adjacent to each other is increased and the number of guard bands is increased as well (refer to FIG. 1). As a result, spectral efficiency is degraded and transmission capacity of the whole system is decreased.
Therefore, as illustrated in FIG. 2B, optical signals of the same modulation method have to be placed on adjacent wavelength positions by rearranging wavelength positions of fragmented optical signal. Accordingly, the number of guard bands can be reduced and optical signals can be arranged closer in a wavelength band, being able to enhance spectral efficiency. By arranging more optical signals in a band which becomes available by the enhancement of the usage efficiency of the wavelength band, transmission capacity can be increased. However, the configuration for the enhancement of the usage efficiency has not concretely proposed in related art.
In related art, a wavelength group wavelength converter which demultiplexes a wavelength multiplexed signal for each wavelength and converts wavelengths of each wavelength so as to multiplex the wavelengths and the configuration including a wavelength group converting device including the wavelength group wavelength converter, the configuration that switches a wavelength of a variable wavelength transponder, the configuration that changes wavelength arrangement so as to make four-wave mixing crosstalk equal to or lower than a predetermined value, and the like are disclosed. Japanese Laid-open Patent Publication Nos. 2002-315027, 2005-286736, and 8-97771 are examples of related art.